Work Log by Kevin Beason

Added Ashikhmin & Shirley’s Anisotropic Phong BRDF and Schlick’s BRDF (the ’94 paper) support for pure path tracing (implicit light paths only). My Schlick BRDF is probably broken. Adding these entailed abstracting a BRDF interface class, implementing importance sampling and computing BRDF/PDF. The daylight image is after sunset for a good reason. The sun would only contribute if a ray found it by chance… talk about noise.

Front row is Ashikhmin & Shirley Anisotropic Phong BRDF. 2nd row is Schlick’s anisotropic BRDF. Third row is my preexisting set of materials. Back row is A&S BRDF with measured reflectance values (in RGB. See previous post.) The floor is A&S BRDF and my fascimile of their texture.

Left image is Preetham daylight model. Right is Debevec’s Uffizi HDR environment probe.

2048 spp, 24-30 minutes each on a Q6600 2.4GHz Core 2 Quad.

This work is from May 2008.

Chromium, Copper, Iron, Lithium, Nickel, Silicon, Tungsten

Inspired by this beauty I set out for rendering with “measured reflectance” values.

FreeSnell comes with spectral data for a variety of materials in a series of .nk files, including the index of refraction (n), the extinction coefficient (k, aka absorbtion), and most importantly in this case, the reflectance. I converted the spectral reflectance into RGB by multiplying by a 6500 K blackbody emission spectrum, converting to XYZ, then to RGB, and normalizing by dividing by the red channel of a 6500 K spectrum converted to RGB. RGB values and code are below. The Spectrum class is in the pane source code. This is work from May 2008.

void nkToRefl(const char *file){
  ifstream inFile;
  inFile.open(file);
  if (!inFile){
    cerr << "Error opening " << file << "\n";
    return;
  }
  string str;
  getline(inFile, str);
  getline(inFile, str);
  fprintf(stderr, "%8s %30s reflectance = ", file, str.substr(18).c_str());
 
  double eV, n, k, r;
  double h = 6.626068e-34;       // m^2 kg/s
  double c = 299792458;          // m/2
  double ec = 1.60217646e-19;    // C
 
  map reflsMap;
  while (!inFile.eof()){
    inFile >> eV >> n >> k >> r;
    double nrg = eV * ec;          // V = J/C => J = VC
    double wavelen = h*c/nrg;      // nrg = h*c/l  => l=h*c/nrg
    double nm = wavelen*1e9;
    reflsMap[nm] = r;
  }
  inFile.close();
 
  vector wavelens, refls;
  for (map::iterator i=reflsMap.begin(); i!=reflsMap.end(); i++){
    wavelens.push_back(i->first);
    refls.push_back(i->second);
  }
 
  Spectrum lightSpec(6500);                // start out with light
  Spectrum reflSpec(&wavelens[0], &refls[0], wavelens.size());
  lightSpec.scaleSpec(1/4.73223e+06);      // normalize by red channel
  reflSpec.scaleSpec(lightSpec);           // mult light by refl
  SbColor rgb = reflSpec.getRGB();
  fprintf(stderr, "(%5f, %5f, %5f)\n", rgb[0], rgb[1], rgb[2]);
}